The evil eye is a widely distributed element of folklore, in which it is believed that the envy elicited by the good luck of fortunate people may result in their misfortune, whether it is envy of material possessions including livestock, or of beauty, health, or offspring. The perception of the nature of the phenomenon, its causes, and possible protective measures, varies between different cultures. In some forms, it is the belief that some people can bestow a curse on victims by the malevolent gaze of their magical eye. The most common form, however, attributes the cause to envy, with the envious person casting the evil eye doing so unintentionally. Also the effects on victims vary. Some cultures report afflictions with bad luck; others believe the evil eye can cause disease, wasting away, and even death. In most cultures, the primary victims are thought to be babies and young children, because they are so often praised and commented upon by strangers or by childless women. The late UC Berkeley professor of folklore Alan Dundes has explored the beliefs of many cultures and found a commonality that the evil caused by the gaze is specifically connected to symptoms of drying, desiccation, withering, and dehydration, that its cure is related to moistness, and that the immunity from the evil eye that fishes have in some cultures is related to the fact that they are always wet. His essay "Wet and Dry: The Evil Eye" is a standard text on the subject.

In many forms of the evil-eye belief, a person otherwise not malefic in any way can harm adults, children, livestock, or a possession, simply by looking at them with envy. The word "evil" can be seen as somewhat misleading in this context, because it suggests that someone has intentionally "cursed" the victim. A better understanding of the term "evil eye" can be gained from the old English and Scottish word for casting the evil eye, namely "overlooking," implying that the gaze has remained focused on the coveted object, person, or animal for too long.

While some cultures hold that the evil eye is an involuntary jinx cast unintentionally by people unlucky enough to be cursed with the power to bestow it by their gaze, others hold that, while perhaps not strictly voluntary, the power is called forth by the sin of envy. In Jewish religious thought, it is sometimes asserted that the one who looks upon another with envy is not always at fault, but that the envy may be perceived by God, who then may redress the balance between two people by bringing the higher one low. It has been suggested that the term covet (to eye enviously) in the tenth Commandment refers to casting the evil eye, rather than to simple desire or envy.

A supposed power bestowed on a person whereby his/her glance brings sickness, bad luck, calamity or death.

In witchcraft and black magic it is said that certain individuals have the power to cast evil spells or to project evil thought forms simply by looking at another person. The idea of this evil power is practically universal, and there exists in virtually every language an comparable term the boser Blick in German, malocchio in Italian, mauvais veil in French; and from the Latin fascinum, which was originally associated with the idea of binding, is derived the English 'fascinate', which was originally connected with such ideas as binding by means of diabolical powers or pact.

The fact of the evil eye has given rise to numerous protecting devices against it. These incorporate a wide range of magical signs and amulets, reflective surfaces, and, in particular, a number of obscene or phallic figures and amulets which are intended to ward off evil such as the corno, a curved horn, and the peculiar gesture involving a clenched hand with the thumb stuck through the middle and fourth fingers. Images of eyes are also used to avoid evil (see eye of horus), on the grounds of sympathetic magic, and many of the more ancient gems and symbols are designed with this in mind.